Theoretical Analysis of ESA-Enhanced 2.8 μm Lasing in Er-Doped ZBLAN Fiber Lasers.

A detailed analysis of theexcited state absorption (ESA) transition 4I13/2→4I9/2 which occurs in erbium (Er) doped ZBLAN fiber lasers is performed through numerical modeling. It is found that the ESA plays a significant role in enhancing the 2.8 μm lasing of Er3+ ions. The ESA promotes the ions residing in 4I13/2 (the lower state of the 2.8 μm laser) to 4I9/2, where the ions subsequently decay to 4I11/2 (the upper state of the 2.8 μm laser) rapidly. An energy recycling is consequently realized via the ESA. The ESA can be activated in cascaded 2.8 μm/1.6 μm Er-doped ZBLAN fiber lasers by absorbing the 1.6 μm signal. Through numerical simulation it is found that in cascaded lasers the slope efficiency of the 2.8 μm laser can be elevated to ∼48% with the assistance of the ESA, a value that surpasses the Stokes limit (∼35%) substantially and agrees well with the experimental results previously reported. It is also found that the ESA-induced energy recycling is exothermic, an effect that is non-ignorable in the high-power circumstance. Moreover, the injection of an additional 1.6 μm pump for directly exploiting the 4I13/2→4I9/2 ESA is simulated using the model. It is found that as the second pump is introduced, the 2.8 μm lasing almost entirely relies on the ESA-induced energy recycling, and hence, the laser is capable of operating under very low 976 nm pump power. The simulation results indicate that the 2.8 μm lasing in Er-doped ZBLAN fiber lasers can even be realized using a single 1.6 μm pump, via a combination of the 4I15/2→4I13/2 and 4I13/2→4I9/2 transition. The outcomes of this work can inspire the development of novel, high-performance 2.8 μm Er-doped ZBLAN fiber lasers. [ABSTRACT FROM AUTHOR]